Electrically connecting of lead or bonding wires to a semi-conductor chip or die mounted on a lead frame or substrate for coupling to external circuitry is generally accomplished by “ball/wedge” bonding. According to this technique, a lead wire or bonding wire 11 is held in a capillary tool 12 so that the wire 11 projects beyond the end of the capillary tool 12 as shown in FIG. 1. The capillary tool 12 forms part of a ball bonding machine in which the tool is appropriately mounted and positioned over the metallized die pad 15 of an integrated circuit chip or other substrate. As shown in FIG. 1, a ball 17 of metal is formed at the end of the wire 11 by melting for example with an electrode 13. This procedure is sometimes referred to as “flame-off”.
After solidification, the metal ball 17 at the end of the wire is brought into intimate contact with the metallized die pad 15 as shown in FIG. 2. A bond is formed typically by thermo-compression bonding applying a specified force and temperature for a specified period of time. Metallic welding and diffusion combine to form this basic bond. Alternatively, ultrasonic bonding or other form of energy may be used. The capillary tool 12 and substrate are then moved relative to each other for bonding of the wire at another location such as for example a finger at the lead frame or on the substrate. At this location, a wedge bond between the lead wire 11 and the finger is generally formed and the wire 11 is severed below the bonding tool. In this manner a lead wire connection is established between the metallized die pad of a chip and the lead frame or the substrate for coupling to external circuitry.
Ball bonding is the preferred method for welding lead or bonding wires to the die pad of integrated circuit chips because the ball can tolerate a greater range of bonding parameters without weakening the wire and furthermore, the lead or bonding wires can be led in any direction from the symmetrical weld. A number of problems are encountered in ball formation however which have generally limited its application to the use of relatively expensive gold lead wires and bonding wires. The primary difficulty in applying the ball bonding method to, for example copper wire and aluminum wire occurs during ball formation. The tip of the wire is melted either by a hydrogen gas torch or by arc discharge between the tip of the wire and a suitably placed electrode. However, during ball melting and formation, the copper or other reactive metal wire oxidizes and the resulting oxide film prevents or interferes in the subsequent ball weld to the die pad. Oxidation also prevents uniform quality ball formation. As a result, the ball bonding technique has generally been limited to the use of gold wires.
The wire used in such ball-bonding processes may be a non-reactive metal such as gold, or a more reactive metal such as copper, silver, palladium or aluminum. When reactive metals such as copper or aluminum are melted in air, they may react with oxygen to form oxides which interfere with bonding. It is therefore desirable to provide a protective cover gas which does not react with the metal around the molten ball, at least until the surface has solidified and cooled sufficiently to become non-reactive. Therefore, methods and apparatus have been developed for providing such a cover gas in which a moveable shroud or shield moves into position before ball formation. The shroud is then filled with a cover gas and the ball is formed at the end of a capillary tool. The shroud is then removed, and the ball-bonding process is completed. For example, U.S. Pat. No. 6,234,376, as shown in FIG. 3, discloses an apparatus for supplying a cover gas to a ball-bonding assembly to protect the molten ball from the effects of exposure to air. The apparatus comprises a gas-containment tube 20 for receiving a shielding gas, the tube 20 having transverse in-line orifices 22, 24 through which the capillary head 16 of the ball-bonding machine can pass. The electrode 30 of an electric flame-off (EFO) device is positioned in the tube 20 such that when the capillary head 16 is in a first position and upon energizing of the EFO, an arc discharge can be formed between the electrode head 32 and the end of a bonding wire which is fed through the capillary head, thereby forming a molten ball 17 at the end of the wire 11.
However, such apparatuses require complex movement of the shroud relative to the capillary tool, requiring control equipment and adding steps to the bonding process. Furthermore, the rapid removal of the shroud after ball formation causes a sudden rush of air to impinge on the hot wire ball. The air can cause surface oxidation of reactive metals, as well as uneven cooling of both reactive and non-reactive metals. Furthermore, the open-ended shroud or tube requires a relatively large amount of gas to maintain the cover gas during the ball formation at the capillary tool and fails to provide a desired flow field of the cover gas around ball and the capillary tool during the ball-bonding process.